The present invention relates to the field of assessing the amount of certain types of particles in a volume of aerosol. Specifically, the invention rapidly senses and reports the concentration of elements having an atomic number greater than about 15, that are suspended in air as solid dust or droplets. The inventive device and method is able to detect various atomic species either singly or collectively regardless of their state of chemical combination. The application of this invention is intended to safeguard workers in industrial plants and to provide data for many types of process control.
The following approaches are used in testing air, but differ from the instant invention.
Gravimetric: The simplest quantitative approach to concentration determination consists of passing a given volume of air through a filter, capturing the transiting particles, and then measuring the filter's mass gain. In cases where the identity of the contaminant is either unimportant or is certain, this technique can be effective. One must however, be careful that extraneous contributors to the rather small mass gain, like water vapor, are very carefully controlled. This condition often precludes both rapid measurement and routine field use, thereby severely limiting gravimetric methods in application which are alarms.
Optical: Perhaps the most varied and widespread methods of particulate assessment use changes in the optical properties of the air itself or filters through which known volumes of air have passed. Some common forms of household smoke detectors look for changes in the optical transparency of an air column to indicate smoke. Other devices, used for testing the carburation of furnaces, collect the smoke on a filter and measure its blackening either by eye or by a photoelectric device. Both of these methods, although simple and rapid, are incapable of any identification of the contaminant. In cases where a harmful ingredient may be masked by the presence of large quantities of harmless smoke, identification is critical. One example of concern occurs often in soldering, wherein lead and cadmium particles from solders and brazing alloys frequently become aerosolized in a thick flux residue cloud. Such problems can be solved by directing a collimated beam of monochromatic ultraviolet or visible light through a vaporized sample of the particulates. As the wavelength of the light is varied it is absorbed according to a spectral pattern characteristic of each species present. The intensity of light at one or more wavelengths absorbed by a given compound is monitored and any decrease used to signal the presence of the compound. Another method, which is basically the inverse of optical absorption, incorporates vaporized particulates into an electrical discharge and studies the optical emission line pattern that results. Both absorption and emission techniques are extremely sensitive and are specific not only to the elemental species, but often even to its state of chemical combination. Unfortunately, both methods also usually involve intricate apparatus, very elaborate procedures, and difficulties in quantification that makes them unsuited for many field and/or automatic alarm uses.
Electrical: If a strong electric field acts on a small particle and if the particle is neutral, the field induces a net dipole moment on the particle. Any field gradients subject such aerosolized dipoles to a net force. The motion evoked under intense gradients adds to the Brownian motion of the particles and produces a net particle drift. This phenomena has long been used as a means to precipitate particles out of industrial aerosols. More recently electrical signals associated with recombination as the particles strike metal surfaces have been used both to herald particle arrival and also to acquire some information about their aerodynamic geometry.
The same functions of detection and aerodynamic characterization can also be performed in a straightforward manner in circumstances where the particulates can be given a net electrical charge. The required net charge may be added to particles either by triboelectric means or by a radioactive source. Once charged, the particle is admitted to a region of strong uniform electric field. The time required for the particle to traverse a given distance is next assessed and from it both the drift velocity and the aerodynamic size may be inferred. Neither electrical method provides direct composition information.
Ionization: Another type of aerosolized particle sensor, which in some ways is similar to the electrical types, is frequently used in household fire protection monitors. In it the detailed electrical behavior of the gas stream is observed as it undergoes bombardment with alpha particles from a radioactive source. Fast moving alpha particles ionize any media through which they pass, including air. If a particulate is introduced, a change in the ionization fraction occurs, a change that can be readily detected. This method is not only simple and inexpensive, but can be made quite sensitive. It provides neither geometrical information nor data on the chemical species present.
Chemical: Chemical means are most often applied to vapors and only occasionally used in particulate sensing. In most cases, one exposes samples of collected particles or the flow stream to chemical indicators that exhibit a pronounced physical change when the target material is present. The extent of that change is then measured and correlated with abundance. Depending upon the specific indicator used, one can sense either elemental materials, ions, specific compounds or entire classes of each. Chemical means are conceptually simple and usually sensitive. Unfortunately, however, chemical methods, like their optical counterparts, are limited to a narrow range of compounds, and often are based on physical effects that are difficult to make quantitative, such as color changes.
Special Methods: Many materials have a unique property, i.e. radioactivity, luminescence, or ferromagnetism, that may be exploited as an indicator. An example is airborne depleted uranium (DU) dust which is detected by scrutinizing collected samples for alpha particle emission arising from uranium's natural radioactivity. Such analysis is of course very slow and subject to the masking effects of naturally occurring radioisotopes.
Clearly then, improved means for aerosol detection which would avoid the above mentioned and other problems, would be worthwhile.